This invention relates to on-board diagnostics for the refrigerant compressor of an automotive HVAC system, and more particularly to a diagnostic method that utilizes existing sensor information.
A significant aspect of state-of-the-art automotive engine control pertains to so-called on-board-diagnosis of various engine components or sensors, particularly when improper operation of such components or sensors can adversely influence the engine emission controls. In the case of a vehicle heating, ventilation and air-conditioning (HVAC) system that includes an clutch-driven refrigerant compressor and an electrically activated clutch mechanism, the diagnosis involves determining whether the compressor is on-normal, on-abnormal, off-normal, or off-abnormal. The on-normal condition indicates that the compressor is actually on (running) when the clutch is commanded on, and the off-normal condition indicates that the compressor is actually off when the clutch is commanded off. The off-abnormal condition indicates that the compressor is not running properly when the clutch is commanded on, and the on-abnormal condition indicates that the compressor is running when the clutch is commanded off. In addition to catastrophic failures such as a seized compressor, the abnormal conditions may be due to clutch failure or slippage, or a control unit failure. As with any diagnostic evaluation, the abnormal diagnostic indications may be used to trigger a check engine lamp or other driver alert so that the detected faulty operation can be corrected.
While the above-described conditions may be logically diagnosed by comparing the compressor speed with the clutch command, a measure of the compressor speed is not ordinarily available. Adding a compressor speed sensor for this purpose would significantly increase system cost, and is therefore undesirable. Accordingly, what is needed is a method of diagnosing the compressor condition without adding a special-purpose sensor.
The present invention is directed to an improved method for diagnosing the condition of an automotive HVAC refrigerant compressor based on information contained within the signal produced by a high-side pressure sensor provided for system control purposes. According to this invention, the output signal of the pressure sensor is divided into its DC and AC components, with the DC component being used for system control purposes, and the AC component being used for diagnosing the condition of the compressor. Pulsations of the AC component (which conventionally are removed by filtering for control purposes) correspond to individual piston stroke cycles. The pulsations indicate compressor operation and are counted for purposes of determining the actual compressor speed.
Since existing pressure sensor information is utilized to verify compressor operation and to determine compressor speed, the system cost impact due to the diagnostic evaluation is minimal. Moreover, the pulsations provide failure information that could not be detected by a speed sensor since the pulsations verify that work is actually being performed by the compressor.